The present invention relates to improved fiber reinforced polyoxymethylene molding compositions. More specifically, this invention relates to fiber reinforced polyoxymethylene molding compositions which form composite articles of improved physical properties.
As is well known, polyoxymethylene, or polyacetal, is a thermoplastic resin which finds wide utility in the manufacture of molded articles. Molded articles of exceptional strength and toughness are obtained when the polyoxymethylene resin is intimately combined with various reinforcing agents. It has been found, however, that various additives are required to provide the desired physical properties to the molded articles prepared from the reinforced polyoxymethylene polymers. Inferior physical properties commonly can be traced to poor adhesion between the polyoxymethylene polymer and the fibrous reinforcement.
Thus, for example, reinforced polyoxymethylene polymers as described in U.S. Pat. No. 3,455,867 provide increased strength in molded articles through the use of chemical coupling agents.
Polyoxymethylene compositions containing certain carbodiimides are disclosed in British Pat. No. 993,600 where the carbodiimides primarily serve an anti-aging role. The patent broadly suggests the use of mono- and polycarbodiimides, but the specific working examples are limited to the use of severely hindered carbodiimides such as 2,6,2',6'-tetraisopropyldiphenylcarbodiimide and the polycarbodiimide of 1,3,5-triisopropyl-benzene-2,4-diisocyanate.
U.S. Pat. No. 3,901,846 discloses that the incorporation of small amounts of specific high molecular weight phenoxy resins in intimate mixtures of the polyoxymethylene polymers and reinforcing agents provides the desired improved physical properties as well as improved surface effects in molded articles.
The non-analogous use of carbodiimides in the preparation of polyoxymethylene polymers is also disclosed in U.S. Pat. Nos. 3,170,896 and 3,135,718.
In the past certain polycarbodiimides have been employed as heat and hydrolysis stabilizers for polyesters as well as in a variety of other areas as illustrated in U.S. Pat. Nos. 3,193,522; 3,193,523; 3,296,190; 3,575,931; and 3,835,098; U.S. Ser. No. 715,946, filed Aug. 19, 1976 now U.S. Pat. No. 4,071,503,Jan.31, 1978, U.S. Ser. No. 753,384, filed Dec. 22, 1976; British Pat. Nos. 1,056,202; 1,231,975; and 1,330,036; Japanese Document No. 75-00044 (summarized in Chemical Abstract, 172327W, Vol. 82, 1975); Belgian Pat. No. 626,176 (summarized in Chemical Abstract, 2054f, Vol. 61, 1964); and Preparation of Carbodiimides from Isocyanates, by W. Neumann and P. Fischer, 1 Agnew. Chem. Internat. Edit. 625 (1962).
In light of this prior art, it is an object of the present invention to provide an improved reinforced polyoxymethylene molding composition capable of forming composite articles exhibiting improved physical properties.
It is another object of this invention to provide reinforced polyoxymethylene composite articles having physical properties which are superior to those of the phenoxy modified fiber reinforced polyoxymethylene articles of the prior art.
It is yet another object of this invention to provide polyoxymethylene molded articles of improved physcial properties employing a specifically defined class of polycarbodiimide not employed heretofore in the art to modify the reinforced polyoxymethylene molding resins.
These and other objects of the invention will become apparent from the following summary and description of preferred embodiments.